1. Field of the Invention
The present invention concerns a heat-developable photosensitive material.
2. Description of the Related Art
In recent years, it has been strongly demanded to decrease the volume of processing liquid wastes in the medical field from the viewpoint of environmental protection and space saving. Thus, technology for photosensitive heat-developable materials for use in medical diagnosis and photographic applications which are capable of being exposed efficiently by laser image setters or laser imagers and are capable of forming clear black images having high resolution and sharpness is required. Such photosensitive heat-developable materials can eliminate the use of solution-based processing chemicals and can provide customers with a thermal development processing system which is simple and does not harm the environment.
While such requirements also exist in the field of general image forming materials, images for medical use particularly require high image quality of excellent sharpness and graininess since fine expression is needed, and images of cold black tone are preferred to facilitate diagnosis. At present, various kinds of hard copy systems that utilize pigments and dyes such as ink jet printers or electrophotographic systems have been marketed as conventional image forming systems, but they are not satisfactory as image output systems for medical use.
Thermal image forming systems utilizing organic silver salts are described in various documents (for example, refer to U.S. Pat. Nos. 3,152,904, 3,457,075, and “Thermally Processed Silver System” written by D. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Walworth, and A. Shepp, Chapter 9, page 279, 1989). In particular, a heat-developable photosensitive material generally has an image forming layer in which a catalytically active amount of photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and, if necessary, a color toning agent for controlling the tone of silver are dispersed in a binder matrix. The heat-developable photosensitive material, when heated to a high temperature (for example, 80° C. or higher) after imagewise exposure, forms black silver images by redox reaction between a silver halide or reducible silver salt (functioning as an oxidizer) and a reducing agent. The redox reaction is promoted by the catalytic effect of latent images of the silver halide formed by exposure. Accordingly, black silver images are formed in exposed regions (for example, refer to U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924). Fuji Medical Dry Imager FM-DP L has been sold as an image forming system for medical use utilizing a heat-developable photosensitive material.
Production of thermal image forming systems utilizing an organic silver salt includes a method of production by solvent coating and a method of production by coating and drying a coating liquid containing, as a water dispersion, fine polymer particles as a main binder. Since the latter method does not require steps such as recovery of the solvent, production facilities are simple, and the method is advantageous for mass production.
In either the solvent coating or the water-based coating method, stable images not dependent upon the material storing conditions can be formed by forming layers including a protection layer on the outer surface of the image forming layer. Further, a photosensitive material having good damge-resistance and transportability in the handling of the photosensitive material during or after processing can be prepared by coating with a protection layer. Further, in view of economical production, it is preferable that two or more layers can be simultaneously coated in a stack.
When a polymer derived from animal protein (for example, gelatin) is used in an outermost layer (for example, refer to JP-A No. 2002-162712), water proofness and storage stability when the photosensitive material was stored over time and under high temperature and high humidity conditions were not sufficient. Further, it has been found that when photosensitive materials are processed to obtain images and then the images are stored in a stack, this results in a problem of color transfer. Further it has also been found that when outputted images are stored in a dark place being stacked upon each other for a long time, this results in a problem of uneven density of images.
The advantage of using gelatin as a binder is that a coated surface can be formed uniformly because of the setting property. Setting means elimination of fluidity by cooling a heated coating liquid just after coating on a support by utilizing the fact that an aqueous gelatin solution undergoes temperature dependent sol-gel change in which it becomes sol when heated to a temperature of 30° C. or higher and gels to lose the fluidity when the temperature is lowered to less than the level described above.
On the other hand, when a coating liquid mainly comprising a latex is applied to form an outermost layer (for example, refer to JP-A Nos. 2000-227643 and 2001-194744), it has been found that while a film of excellent water proofness can be formed, the stability of the coating liquid for the outermost layer highly tends to be affected by a salt concentration in additives of an adjacent layer, pH, or surface static charges of dispersed particles because of the lack of the setting property and creases sometimes occur on the surface of the photosensitive material during drying of the coating.
In view of improving the water proofness and the storage stability when the photosensitive material is stored over time and under high temperature and high humidity conditions, it is known that a hydrophobic polymer is preferably used as the binder for the outermost layer, but gelatin has often been used in consideration of problems in the production step.
Further, it has not yet been studied sufficiently as to what polymer is effective for use as the hydrophobic polymer, and the effect of changing the polymer species could not be anticipated.
Accordingly, a heat-developable photosensitive material having good storage stability before image formation and less fluctuation of sensitivity and also having excellent image storability in a dark place after image formation is required.